Editors’ Highlights

What Makes a Terrestrial Gamma-Ray Flash in Thunderclouds?

Two lightning flashes were observed in the same location: One produced a bright gamma-ray flash with about 1000 counts per millisecond, but the other did not.

Source:
Journal of Geophysical Research: Atmospheres

Two lighting channels that occurred four hours apart recorded by a 30,000 frame-per-second camera. The lightning on the left did not produce a gamma-ray whereas the one on the right produced a bright gamma ray. Credit: Smith et al., 2018, Figure 2, right panels

Bright flashes of high-energy radiation (gamma rays) have been seen along with some lightning, usually beamed upwards into space. These “terrestrial gamma-ray flashes” (TGFs) are so bright that someone caught in the middle of one could get a hazardous radiation dose, but that region is generally in the middle of a thundercloud where airplanes seldom go. Smith et al. [2018] report two very similar – but unusual – lightning flashes that went upward into the clouds from the top of a tower in Japan in the winter. One produced an enormously bright downward TGF that paralyzed detectors on the ground while the other produced none at all. The authors present evidence that the TGF occurred in the first case because the electric field was higher than it was during the lightning that didn’t produce radiation.

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